1. Field of the Invention
The present invention generally relates to an illumination system, and more particularly to an illumination system adapted to be used on a projection apparatus.
2. Description of Related Art
Referring to FIG. 1, a conventional projection apparatus 200 includes an illumination system 100, a light valve 210 and a projection lens 220. The illumination system 100 includes a laser light source 110, a beam expander 120, an integration rod. 130 and two focusing lenses 142 and 144. The laser light source 110 is used to provide an illumination beam 112, and the beam expander 120 is used to expand the illumination beam 112. The focusing lens 142 is used for focusing the illumination beam 112 on the integration rod 130. The integration rod 130 is used for uniforming the illumination beam 112, and the focusing lens 144 is used for focusing the illumination beam 112 from the integration rod 130 on the light valve 210. In addition, the focusing lens 144 also forms an image of a light exit end 132 of the integration rod 130 on the light valve 210. Furthermore, the light valve 210 converts the illumination beam 112 into an image beam 112′, and the projection lens 220 projects the image beam 112′ on a screen 50, so that an image is formed on the screen 50.
Because the illumination beam 112 provided by the laser light source 110 is a highly coherent illumination beam, diffraction and interference of the illumination beam may easily occur. This often leads to a speckle phenomenon in the image on the screen 50, thereby affecting the imaging quality. In the prior art, a number of methods for lowering the coherence of the illumination beam 112 are used to reduce the degree of the speckle phenomenon. For example, in U.S. Pat. No. 7,030,383, the coherence of the illumination beam 112 is lowered to reduce the degree of the speckle phenomenon by rotating the focusing lens 144 in the direction of the arrow 42 or moving the focusing lens 144 in the direction of the arrow 44.
However, the focusing lens 144 forms the image of the exit end 132 of the integration rod 130 on the light valve 210, such that moving or rotating the focusing lens 144 makes the image of the exit end 132 of the integration rod 130 not formed on the light valve 210 accurately, which adversely affects the imaging quality of the projection apparatus 200.